1. Field of the Invention
The present invention relates to gaskets and more particularly to a rubber-coated gasket which includes a metal sheet having a rubber layer coated on one or both sides thereof. The rubbercoated gasket has such a high resistance to antifreezes that even if the gasket is exposed to an antifreeze, the rubber coating(s) will not separate from the metal sheet.
2. Description of The Related Art
Rubber-coated gaskets for use in various apparatuses and machines are known, generally, and are typically fabricated by punching or stamping desired shapes out of sheeting that consists of a metallic substrate such as a sheet of carbon steel, stainless steel or aluminum, with a rubber coating formed thereon. Rubber-coated gaskets have several advantages. In particular, because the substrate is formed from a high-strength metal sheet, the gasket will not allow any leakage of the internal gas under pressure. Further, the elastic surface rubber layer enables the gasket to closely fit the irregularities which may be found in the surface of a flange and thereby insures reliable gas sealing properties.
The structure of the a rubber-coated gasket of the type to which the present invention relates is shown in cross-section in FIG. 1. The prior art process for manufacturing such a gasket includes a series of steps. First, the metal sheet 1 is degreased and is then roughen by a suitable technique such as shot blasting or Scotch brightening. A chemical conversion coating 2 is then formed on the metal surface. A primer coating 3 is then formed as an adhesive layer by applying a primer based on a phenolic resin. The primer layer 3 is then coated with a given thickness of a rubber solution which is a mixture of nitrile rubber with a reinforcing carbon black filler, as well as a curing agent and other additive commonly used in rubbers. The coating solution is then dried and cured to form a rubber layer 4. Finally, depending upon the operating conditions of the gasket, a nonsticky layer 5 is formed on the rubber layer 4 by coating a dispersion of graphite, molybdenum disulfide, etc. (See for example U.S. Pat. No. 2,957,784).
As noted above, the convention process for forming rubber-coated gaskets includes using a phenolic resin based primer layer 3 which is overlaid with the nitrile rubber layer 4 using carbon black as a reinforcing filler. Since these gaskets have a high resistance to oils and Freon gases, they can be used with satisfactory results as gaskets for example in a compressor of a refrigerator. However, in automotive applications which is one of the more recently proposed uses of such gaskets, the primer layer 3 is corroded by hot antifreeze and the rubber layer 4 can separate from metal sheet 1. There are three major causes of this phenomenon. First, antifreezes are chiefly composed of ethylene glycol which is commonly used as a coating stripping agent. In addition, amines, phosphates and other compounds that are incorporated in antifreezes to, for example, inhibit corrosion, have a corrosive effect on the primer. Finally, antifreezes which are adjusted to a weekly alkaline state have, a pH of 8-9. However, the phenolic resin which is the chief component of the primer is decomposed by hot alkalines. Therefore, the phenolic resin can be decomposed by the hot antifreeze.